Biomass-based fuels are considered carbon neutral and are generally regarded as a renewable energy source. With the abundant availability of biomass waste materials globally, converting such materials into a fuel in form of raw pellets, pyrolysis oil (bio oil), cellulosic ethanol, biogas or gasification products has been the subject of intensive research. These processes are intended to increase the energy density of the biomass waste material so that they can be transported economically to markets and readily used by conventional combustion based power generation technologies.
Torrefaction describes a process in which biomass materials are heated to between 200°-320° C. in absence of oxygen, to completely drive out the moisture within the biomass materials, as well as some low-boiling-point volatiles, so that the biomass material is amenable to further compression into high energy density pellets. Torrefied biomass pellets have the added advantage of being slightly hydrophobic, so they do not absorb water that can cause swelling and disintegration or subsequent biodegradation through the attack by microbes and fungi. Thus, torrefied biomass fuel is stable enough to be shipped across long distances and to be stored in silos for long period of time. These are important considerations for a commodity solid fuel.
Additionally, use of torrefied biomass pellets allows the possibility of co-firing in coal based utility power stations with minimum retrofit, while significantly reducing the carbon footprint of the power station. Since coal fired power stations often have the highest carbon emission profiles of all power generation technologies, the operators of such stations have been requested by various governments to take measures to reduce their carbon emissions. For example, the European Union has proposed up to 25% co-firing of biomass pellets to achieve meaningful carbon emission reduction targets without compromising the operations of the utilities.
Numerous methods have been proposed for meeting the heating requirements for torrefaction of biomass materials. For example, use of fluidised beds, moving beds, screw reactors, and shaft furnaces have been proposed for use in torrefaction. However, since one of the major considerations is to keep the torrefaction reactor oxygen-free, such methods may face significant challenges, since they all need gas as the fluidising medium and/or as the convective heat transfer medium. Air would generally not be used as such a medium for the torrefaction process, since it contains approximately 21% oxygen, and torrefaction should be carried out in a substantially oxygen-free environment. The cost of using substitutes for air as the fluidising medium and/or convective heat transfer medium could be prohibitive. Other proposals involve use of rotary furnaces for torrefaction of biomass materials.
Of course, in addition to the “usual” challenge of maintaining low- or no-oxygen conditions during torrefaction, a relatively constant operating temperature should be maintained. It can be difficult to achieve and maintain such a constant operating temperature if hot gas or a fuel-based heating system is used. Additionally, previously proposed torrefaction methods may face difficulties with generally poor heat transfer to loosely-packed biomass material, which may slow the heating process and make it less efficient. Due to economic considerations, it is generally desirable to keep the process time to less than 30 minutes.
Further, it would be desirable to control the particle size of the biomass material, in order to provide particles that are suitable for formation into pellets during a subsequent pellet making process. Current torrefaction processes are generally designed only to “heat” the biomass material to a desired temperature. Subsequent particle size reduction may be necessary in order to provide particles that are suitable for the production of pellets.